In page 493, one of the three reasons why our calculations overstate the value of interest tax shields.  "Third, you can't use interest tax shields unless there will be future profits to shield." Why? Is there anyone call pick some example to help me understanding it?

[NEAS: If the firm has no profits, it pays no income tax. The debt tax shield reduces its taxes to a minimum of zero, which is no reduction if the tax liability is already zero.

A firm can carry back operating losses two years and carry them forward five years. If the firm has no future profits for several years in a row, the tax shields are worthless.]